1. Field of the Invention
The invention relates to novel alloy specifications from the class of fully martensitic 9-15% chrome steels. By means of a controlled precipitation sequence in the quenching phase, excellent properties and property combinations for wide applications in the power station field can be provided.
2. Discussion of Background
Fully martensitic quenching and tempering steels with 9-12% of chromium are widely used materials in power station engineering. Properties of interest for high-temperature applications are their low manufacturing costs, their low thermal expansion and their high thermal conductivity.
The mechanical properties important for the use are produced by a so-called quenching and tempering process. It is carried out by a solution-annealing treatment, a quenching treatment and a subsequent tempering treatment in a moderate temperature range. The resulting microstructure is distinguished by a dense arrangement of laths with integral precipitation phases. These microstructures are unstable at elevated temperatures. They soften as a function of time, of stress and of the deformations forced on them. The phase reactions proceeding during the heat treatment restrict the achievable ductility within the scope of the demanded strengths. The phase reactions proceeding during operation together with the coarsening of the precipitations cause an increased susceptibility to embrittlement and reduce the expansions to which the components are subjected.
As a consequence of these structural instabilities during the heat treatment and in operation, the current alloys from the class of fully martensitic 9-15% chrome steel no longer meet the requirements of modern power station engineering. This applies primarily to the combination of strength and ductility, and also to combinations of high-temperature strength, creep resistance, creep rupture strength, relaxation strength, resistance to creep embrittlement and thermal fatigue. Narrow metallurgical limits for a steady improvement in the properties of this alloy class are set by the requirement of a capacity for full quenching and tempering, in particular in thick-walled components.
Within the scope of the restricted metallurgical possibilities, further improvements in the properties and property combinations are mainly achieved only if an enhanced stability of the microstructural states being formed in the individual heat treatment phases is obtained by the alloying measures taken. This includes in particular an increased resistance to grain coarsening at increased solution-annealing temperatures, improved hardenability during quenching and increased resistance to softening during the final tempering treatment (tempering resistance).
In the industrially known and newly launched alloys, an optimum combination of grain coarsening resistance, hardenability and tempering resistance is achieved by a suitable (empirical) matching of vanadium, niobium, carbon and nitrogen. Optimum combinations are obtained when the carbon content in atom percent is higher than that of nitrogen. The optimum carbon content is in the range of 0.1-0.2% by weight and the optimum nitrogen content is in the range of 0.05-0.1% by weight. In order to achieve a maximum tempering resistance coupled with a high grain coarsening resistance, nitrogen is alloyed in almost stoichiometric proportions with the alloy nitride formers vanadium or niobium. The optimum content of vanadium is consequently in the range of 0.2-0.35%. by weight and that of niobium is in the range of 0.05-0.4% by weight. The state of the art is well represented by the earlier alloys X22CrMoV121 (X22), X20CrMoV121, X12CrNiMo2, X19CrMoVNbN111 (X19) and by the more recent alloys X10CrMoVNbN91 (P/T91), X12CrMoWVNbN1011 (rotor steel E2), X18CrMoVNbNB91 (rotor steel B2) and by the alloy X20CrMoVNbNB10 1 (TAF).